The invention relates to a decoupling sleeve for mounting in a motor vehicle exhaust pipe.
In general, a motor vehicle engine exhaust pipe is connected to the outlet of the exhaust manifold of the engine. This pipe generally comprises one or more cylindrical units (catalytic converter, expander, muffler) constituting a number of masses along a pipe proper itself made up of one or more rigidly interconnected segments. The assembly is suspended from the bodywork of the vehicle via moderately flexible links formed by suspension points or straps generally based on elastomer, and it must accommodate movements of the engine associated with vertical accelerations, with sudden changes of speed, with thermal expansion, with assembly tolerances, . . . .
That is why an exhaust pipe usually includes a flexible tubular coupling referred to as a xe2x80x9cdecoupling sleevexe2x80x9d which serves to avoid the exhaust pipe being damaged or destroyed by the various constraints mentioned above. The sleeve makes it possible to obtain the flexibility that is required for decoupling engine vibration from the exhaust pipe and from the bodywork, and it improves comfort in the cabin of the vehicle.
As a general rule, the sleeve must withstand a flow of hot gas whose temperature can reach or even exceed 900xc2x0 C. on a continuous basis and it must also withstand the external conditions to which the exhaust pipe is subjected. However, the sleeve must not interfere with the operation of various elements in the exhaust pipe, and in particular, for gasoline engines, it must not allow any air to enter the catalytic converter.
Sleeves are known that are constituted by tubes of 0.3 mm to 0.5 mm thick stainless sheet metal, the sheet metal being corrugated to form a bellows having an internal lining and an external braid or knit. However, such sleeves are stiff and they have modes of vibration that are well-marked and noisy. Furthermore, they can withstand no tension, no twisting, and no deformation in bending, in shear, or in compression in excess of acceptable levels.
In general, the degree of sealing required upstream from the catalytic converter makes it necessary to use a covering that is closed completely and in leakproof manner since otherwise it would be necessary for the elements of a discontinuous structure to be clamped together extremely tightly, which state of affairs is difficult to maintain under all deformations of the sleeve. In addition, deformations of the sleeve give rise to folding and to folds that crisscross, and the continuous tube or sheet elements must be extensible since otherwise they risk mechanical destruction.
Unfortunately, flexible materials capable of providing the required degree of sealing, e.g. elastomers, are of limited temperature capability. It is therefore necessary to have a high temperature elastomer of silicone or analogous type, together with thermal protection.
Difficulties with thermal insulation are then made worse by constraints associated with space availability, since the inside diameter of the sleeve cannot be made smaller without greatly impeding the flow of gases, while its outside diameter is constrained by the very limited amount of space available for the exhaust pipe. Furthermore, conduction through the couplings causes the leakproof junction between them and the elastomer covering to be the hottest region of the covering. Finally, ordinary refractory felts have thermal conductivity that increases greatly at high temperature.
In document FR-A-2 676 502, the decoupling sleeve is made of a silicone type high temperature elastomer material which is protected internally by refractory and fibrous insulating materials. The need for providing at least a minimum level of bending strength requires the silicone to be very thick (about 7 mm thick) which considerably degrades effects relating to thermal protection and to the flexibility that can be expected in compression.
In document EP-A-0 145 020, the decoupling sleeve has a cylindrical metal framework which serves as a support for a set of superposed layers, in particular an inner fabric layer based on inorganic fibers, a sealing layer constituted by a metal foil, a thermal insulation layer, and an outer protective layer. At each end of the sleeve, the set of layers is pinched together by means of stitches made with wire.
In document FR-A-2 758 588, the decoupling sleeve is constituted by a functional stack of materials preferably shaped to form a bellows and comprising an inner layer, a barrier layer, a thermal insulation layer, and a leakproof outer layer made of elastomer, the stack being held between two interleaved helical springs, one on the inside and the other on the outside.
At each of their ends, the two springs present touching turns to constitute rigid surfaces. These surfaces which extend over a length of not less than 1 cm are used to enable the sleeve to be connected to the ends of two pipe segments of the exhaust pipe between which the sleeve is interposed. Such connections can be made by means of rigid collars or by crimping, for example.
However, to obtain good sealing and good mechanical strength simultaneously, all of the layers beneath the elastomer, including the insulation which is already rather thin given the space available, are highly compressed by the crimping and thus lose their effectiveness. In the absence of a sealing gasket, clamping does not provide sufficient sealing for an application to engines having catalytic converters, for example.
The proximity of the connection to the pipe proper, which is generally thin and made of metal, worsens the heating of the elastomer in this zone because of conduction, in particular when continuity is established for sealing purposes.
Given the structure of the prior art decoupling sleeves mentioned above, and the various constraints they are called on to withstand, an object of the invention is to design a decoupling sleeve which does not present the drawbacks of the prior art and which is capable of complying as well as possible the conditions it needs to satisfy in order to mitigate the effects of the constraints inherent to the operating conditions of an exhaust pipe.
In other words, an object of the invention is to design a flexible decoupling sleeve which is capable of withstanding a flow of gas at high temperature, and which is intended more particularly for providing mechanical and vibrational decoupling in an exhaust pipe.
To this end, the invention provides a decoupling sleeve for mounting in a motor vehicle exhaust pipe, the sleeve comprising in particular a mechanical portion having two rigid end zones for connection to the exhaust pipe, and a sealing portion that is thermally insulated from the inside by an insulation portion and that extends over substantially the entire length of the mechanical portion of the sleeve, the sleeve being characterized in that the sealing portion comprises a central zone formed by a layer or wall made of a material that is flexible and that withstands temperature, and two rigid end zones respectively connected to two end zones of the mechanical portion, the sealing portion forming a covering that is continuous, closed, and leakproof.
According to another characteristic of the invention, the mechanical and sealing portions are dissociated from each other so as to perform their respective functions independently of each other.
The flexible layer or wall of the sealing portion of the sleeve is made of an elastomer material such as silicone which is thermally protected radially by the insulation portion and axially by the two end portions of the sealing portion.
The two end portions of the sealing portion are constituted by two long thin covers, preferably made of a material that conducts heat poorly, such as stainless steel for example, said covers serving to reduce heat conduction towards the flexible layer and also to contribute to making the sealing portion in the form of a continuous covering that is closed and leakproof.
In an embodiment of the sealing portion:
the thickness of the flexible wall is about 0.5 mm to 3 mm, and the wall is folded and reinforced by flexible turns, e.g. to provide better mechanical strength against the pressure of a gas flow; and
each cover is relatively long and thin, possibly also having fins, folds, or conductive extensions to provide a greater area for evacuating heat, while the thinness and the poor conductivity of the covers limit heat conduction from the two hot zones where the sleeve is connected to the exhaust pipe.
In general, the radial insulation portion of the flexible wall of the sealing portion extends beneath the covers to insulate them, and it is made of an insulating material having a thickness of about 5 mm to 20 mm.
The mechanical portion of the sleeve can be made with various structures, some of which are described below, it being understood that this mechanical portion does not in itself constitute an essential characteristic of the invention.
In an important advantage of the invention, the fact of connecting the decoupling sleeve to the exhaust pipe via the mechanical portion of the sleeve serves to prevent the thermal insulation and sealing portions being crushed and also serves to limit the amount of heat that is transferred to these portions